Polyvinyl alcohol and its hydrogel forms have a relatively long history of use in a wide variety of applications. Polyvinyl alcohol in the form of fibers and covalently cross linked polyvinyl alcohol sponges and foams have already established themselves as very useful materials in numerous applications such as in packaging, thermal and acoustic insulation, construction, furniture, transportation aerospace, food industry, household, textile, medical, cosmetics, and a number of other areas, For example, polyvinyl alcohol sponges are used commercially as filters for water, air filters in intakes of compressors, engines, and air conditioners, oil filters, and the like. Large numbers of uses of polyvinyl alcohol sponges are based on their ability to readily absorb and hold water such as, household sponges, absorbent cloths, industrial dehydrating rollers, paint rollers, acoustic filters, and the like. Polyvinyl alcohol in the form of fibers is also used in a wide variety of applications.
The use of polyvinyl alcohol hydrogels in the medical field is especially important because of the physico-chemical properties of the hydrogels. When the hydrogels are physically cross linked, they have exceptional compatibility with human and animal tissue. Some of the unique properties of physically cross linked hydrogels is that they are imperviousness to attack by body fluids, blood, urine and other bodily secretions. They are non-sticking and non-adherent to tissue, essentially they do not have an affinity for sticking to proteins and they do not have cell adsorption. They are non-thrombogenic and have exceptional biocompatibility.
There are basically two families of methods for the preparation of bulk and cellular hydrogels, that is, one method which relies on covalent cross linking and the other method which requires physical cross linking of the polyvinyl alcohol molecules.
Thus, covalent cross linking, also known as chemical cross linking, includes the use of multi-functional reactive chemical molecules such as aldehydes, maleic acid, dimethyl urea, di-isocyanates, boric acid, and the like, and also the use of ionizing radiation, ultraviolet light, and the like, while physical cross linking methods, also known as reversible cross linking, includes cross linking through crystallites, hydrogen bonding and complexing agents such as titanium, aluminum, manganese, and copper, to name a few. Physical cross linking through formation of crystallites in polyvinyl alcohols has been reported, using for example, partial freeze-drying, repeated freezing and thawing, low temperature crystallization, physical cross linking induced by the presence of aqueous solutions of organic compounds, salts, acids and bases and the like.
Porous (cellular) polyvinyl alcohol materials have been prepared by frothing methods and the only method known to the inventor herein is the preparation of cellular polyvinyl alcohol hydrogels using covalently cross linked polyvinyl alcohol matrices. Physical cross linking methods have been reported only for the preparation of bulk polyvinyl alcohol hydrogels.
The preparation of cellular polyvinyl alcohol hydrogels having open pores by reacting polyvinyl alcohol with formaldehyde in an aqueous solution has been known for a long time. The earliest disclosure of a method can be found in U.S. Pat. No. 2,609,347, which issued to Wilson in 1952, that teaches the preparation of porous polyvinyl alcohol hydrogels by cross linking the hydrogels with formaldehyde at temperatures between 20° C. and 60° C. in the presence of an acid catalyst, such as sulfuric acid. Porous structures are created by entrapping gas bubbles in the polyvinyl alcohol solution in the presence of wetting agents that stabilize the bubbles and help to disperse the bubbles uniformly throughout the polyvinyl alcohol phase. The first step in the preparation of those hydrogels is the preparation of a solution of the polyvinyl alcohol or its copolymers in appropriate solvent, typically water. Then the entrapment of the air bubbles in the polyvinyl alcohol solution in the presence of a surfactant is carried out and finally, the polyvinyl alcohol is cross linked by reacting it with a multi-functional cross linker.
The cross linking agents used in the prior art processes render the polyvinyl alcohol sponges insoluble in any solvent due to formation of the covalent bonds between the molecules. Typically, cross linking agents for the hydrogels are selected from the aldehyde family such as for example, formaldehyde, glyoxal, gluteraldehyde and others that leads to the formation of highly acetalized cellular networks.
The only method for the preparation of cellular polyvinyl alcohol hydrogels by a pore forming method is that based on chemically cross linked matrices. The inventor herein is not aware of any reported method for the preparation of physically cross linked cellular polyvinyl alcohol hydrogels using pore forming methods.
Bulk polyvinyl hydrogels can be prepared by a number of methods. These methods teach gelling of the hydrogels from their solutions, by, for example, cooling the solution, or by addition of gelling agents such as, for example, phenol, naphtol, Congo Red or amino or metallic compounds. Initially, only aqueous solutions were used and were gelled by cooling to room temperature or below 0° C. Such hydrogels are invariably fragile, weak, sticky and unstable in water. A number of methods have also been reported to enhance the properties of such hydrogels. Almost every time, it was attempted by inducing additional chemical cross links using aldehydes, boric acid, radiation and coordination bonding. However, none of the methods that generate chemical bonds was successful in sufficiently enhancing the physical properties of the hydrogels.
A major improvement in the performance characteristics of the hydrogels is disclosed in U.S. Pat. No. 4,663,358 that issued to Hyon in 1987. This patent discloses a method of manufacturing polyvinyl alcohol hydrogels by cooling a solution of the polyvinyl alcohols to below 0° C. in a mixed solvent consisting of water and a water-miscible organic solvent. The preferred solvent is a mixture of water and dimethylsulfoxide, with the water concentration being in the range of from 10 to 90 weight percent. The hydrogels prepared from mixed solvents are transparent whereas hydrogels prepared from the solution in either water or dimethylsulfoxide as the only solvent, are opaque.
U.S. Pat. No. 4,851,168 that issued in 1989 to Graiver teaches a method of preparation of hydrogels and in particular polyvinyl alcohol fibers, by cooling a non-aqueous solution of polyvinyl alcohol to below −10° C., wherein the solvent is a mixture of monohydric alcohols containing 1 to 4 carbon atoms and dimethylsulfoxide. The preferred concentration of mixed organic solvents is about 10 to 30 weight percent of a monohydric alcohol and the rest being dimethylsulfoxide.
A review of the prior art has disclosed that no references were found for laminated structures or for a structure with composite or composite-like properties, or for impregnated structures of physically cross linked bulk polyvinyl alcohol hydrogels.
Two patents, U.S. Pat. No. 5,336,551 that issued to Graiver in 1995, and U.S. Pat. No. 5,422,050 that issued to Graiver in 1994, teach the composition of matter and the method to reinforce bulk polyvinyl alcohol hydrogels by increasing the degree of crystallinity of PVA hydrogels by imbedding externally preformed crystallites of desired size, shape and aspect ratio In the hydrogels. Column 2, line 65 states that “The objective of the invention is to provide a method for introducing highly crystalline regions into a polyvinyl alcohol hydrogel. The regions function as reinforcing agents for the hydrogel”. This is a very key statement in Gravier's patent. That statement reflects exactly what they have done to improve the tensile properties of PVA hydrogel matrices, that is, by dissolving PVA fibers starting from the outer surface. These treated PVA fibers are imbedded into the PVA hydrogel matrix and fibers become an integral part of the matrix without any identifiable boundaries between the fibers and the hydrogel matrix. In fact, crystalline regions of the fiber become crystalline domains in the new PVA hydrogel matrix. These crystalline regions join the crystallites that have been created in the PVA hydrogel as physical crosslinking sites. They simply increase the degree of crystallinity of the PVA hydrogels.
The reinforcement is accomplished by uniformly dispersing a plurality of fibrils made from highly oriented crystalline polyvinyl alcohol, wherein the diameter of the fibrils is less than 1 mm and the aspect ratio of the fibrils is from 2:1 to 1000:1. The key feature of a reinforced hydrogel material made according to this invention is that it has a gradual transition in the degree of the crystallinity at the interface between the matrix and the fibrils.
As opposed to the prior art structures, the structures of the invention disclosed herein require no prior treatment of the polyvinyl alcohol fiber to establish strong interfaces between the fibers and the hydrogel matrix. This leads to cohesive failure as the only failure mechanism of the reinforced polyvinyl alcohol hydrogels. The imbedded PVA fibers used for reinforcement never become an integral part of the PVA hydrogel matrix but remain and retain their own identity all the time while imbedded into the PVA hydrogel matrix. Also, the present invention requires no pretreatment of a number of other fibers or structures used to reinforce and/or laminate such hydrogels, such as, silk, wool, cellulose, acrylates, carbon, graphite, and the like. The simple addition of these fibers or structures to the polyvinyl alcohol solution prior to gellation or crystallization will provide sufficiently strong interfaces with the hydrogel and thus, ensure no adhesive failures of the structures set forth herein.
By the invention herein, there is provided methods by which a material with composite-like structures can be obtained by combining physically cross linked bulk or cellular polyvinyl alcohol hydrogels with other materials and their structures. One can also combine physically cross linked bulk or cellular polyvinyl alcohol hydrogels with covalently cross linked polyvinyl alcohol hydrogels and arrive at unique unitary structures capable of providing adhesive strength. Such adhesive resistance, wherein any failure is due to cohesive failure, indicates that the interfacial bonding strength is higher than the strength of the polyvinyl alcohol hydrogel itself.
Thus, in summary, the prior art teach the use of polyvinyl alcohol fibrils to reinforce bulk polyvinyl alcohol hydrogels. The method of the prior art requires heating the fibrils in a solvent for a certain time periods to soften and partially dissolve the surfaces of the fibrils that is necessary to impart strong interfaces between the fibrils and the hydrogel. This method is cumbersome and is difficult to use because of the difficulties in defining the exact time necessary to soften the fibrils without over-softening them. Furthermore, any upset in the process parameters, especially an increase in the solvent treatment temperature, or exposure to the solvent for too long a period of time, will lead to excessive or even complete dissolution of the fibrils.